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Topic: lens question for the math geniouses (Read 5187 times)

There's an easy way for a photographer to get a mental grasp on the meaning of focal length.

Take a sheet of cardboard and cut a hole in it the same size as your camera's sensor (or film or whatever). Hold it 50mm away from your eye, and you'll see exactly the same field of view as your camera captures with a 50mm lens. Hold it 24mm away and you see what you would with a 24mm lens. Hold it 50mm away but with a hole cut for a different format and you now see what that other format sees with a 50mm lens.

50mm is about two inches. Space your fingers about an inch and a half apart and hold them about two inches away, and you see what a 135 format ("full frame") camera sees with a 50mm lens. Hold your hand in the same spot but bring your fingers closer, to about an inch apart, and you now see what an APS-C camera does with a 50mm lens. Hold a sheet of paper two inches from your eye and what it covers is what a large format camera sees with a 50mm lens. Hold your little finger two inches from your eye and what the tip covers is what a P&S camera sees with a 50mm lens.

...and now you should understand all you as a photographer need to know about focal length and image format.

For bonus points, imagine a spot midway between your fingers and your eye where the light gets focussed through. That spot is the size of the focal length divided by the aperture. For f/2, it's half the size; for f/4, it's a quarter the size, and so on. Hold your fingers two inches away an inch and a half apart and imagine a 3/4" hole between; that's 50mm f/2. Keep your fingers an inch and a half apart but move them four inches away, but the spot stays at 3/4"; that's 100mm f/4.